Some ISPs offer YouTube/Netflix/Facebook for free. How do they monitor that?

In Poland, it is common for mobile ISPs to offer plans with limited amount of bandwidth per month, with exclusion of some popular apps. So for example all traffic from youtube is not counted towards the data cap, but any other casual everyday browsing is.

Aside from net neutrality issues, I am wondering how is this achieved in the HTTPS age? How does the ISP know which packets to count towards the data cap?

It doesn't seem to be per-app, and no proxy is installed. My guess is that it's done by some kind of packet inspection. But what exactly are they looking at?